1. Technical Field
The present invention relates to a motor control apparatus for controlling a synchronous motor, and more particularly to a motor control apparatus for controlling a motor without using position sensors, and an electric appliance having such a motor control apparatus.
2. Related Art
Hitherto is known a so-called fully automatic washing machine automating all processes from washing to dewatering.
A conventional fully automatic washing machine is, for example, designed to detect the cloth amount (laundry amount) put in a washing-dewatering tank before start of washing in order to carry out the processes smoothly from washing to dewatering. In the conventional detecting method of cloth amount, a Hall element is used to detect a rotational phase of a rotor in the motor, and the motor is driven to a specified speed by detecting rotational phase of the rotor based on a signal from the Hall element. At that time, a motor current is also detected, and the motor current flowing while the motor rotates at predetermined speed is summed. Based on the summed motor current and the inertia time of the motor after the driving signal is stopped and until the motor slows down to a predetermined speed, the cloth amount is detected (see patent document 1).
In other prior art (refer to as “second prior art”), a technique without using position sensors to detect the cloth amount is disclosed. This detects a current flowing through the motor to estimate the rotation phase and rotating speed of the rotor, and detects the cloth amount using the torque axis current component obtained by vector operation (see patent document 2).
A motor control apparatus in a different prior art (refer to as “third prior art”) detects the reactive current supplied in the motor, and controls by feedback so that this value may reach the target value (see patent document 3). The third prior art is explained below by referring to FIG. 24.
FIG. 24 is a block diagram of the motor control apparatus in the third prior art. In the diagram, direct-current voltage of a direct-current power source 701 is converted into alternating-current voltage by an inverter circuit 702, and is supplied to a motor 703 by way of a motor current detector 704.
In an inverter controller 55, a processing unit 58 creates and outputs PWM command from a command value for the motor applied voltage, and controls switching elements of the inverter circuit 702 to drive the motor 703. At this time, the motor current detector 704 detects the current flowing in the motor 703 and outputs a detection signal. A detector 57 calculates a reactive component of the motor current on the basis of the detection signal. A setting unit 56 outputs a rotation frequency command value and a reactive current command value. The processing unit 58 generates applied voltage command by calculation based on the error of reactive current command value and reactive current detection value, generates PWM command value from the applied voltage command to output it to the inverter 702, thereby controlling the inverter circuit 702 again in next control cycle.                Patent document 1: JP 09-253379A        Patent document 2: JP 2002-360970A        Patent document 3: JP 2003-204694A        
The washing machine in the first prior art includes a position sensor for detecting motor driving and cloth amount, but the position sensor such as Hall element is expensive and wiring for the position sensor signal is required, which is contradictory to reduction of size and lowering of cost. Further, the position sensor has a lower allowable temperature of ambient temperature as compared with allowable temperature of the motor. Especially, in the washing-drying machine for operating continuously from washing to drying, the mounting position of the position sensor is limited, and it is a significant restriction when designing the mechanism of the washing-drying machine.
Besides, depending on the position error in mounting the position sensor, the actual phase of the rotor may be different from the phase signal output from the Hall element. In this case, the controller calculates and determines the applied voltage using the wrong phase signal, and thus the motor efficiency may be out of the best point. Hence the motor efficiency may be lowered or the vibration noise may be caused. Further, if the position sensor is broken, the motor cannot be driven at all, and the reliability of the entire washing machine is sacrificed.
The washing machine in the second prior art has no position sensor and thus has an advantage in that problems caused by the position sensor do not occur as described to the first prior art. However in the case of a drum washing machine, during washing operation, laundry is lifted up by the rotating tank and falls down before reaching the top. This washing operation causes one drop of the laundry every rotation of the rotating tank from 90 degrees to 180 degrees, and the drop of the laundry causes a large change of the load which is a large change of the load on the motor rotating the rotating tank. Rotating speed of the tank should be low, because high rotating speed causes the laundry to stick to the rotating tank by centrifugal force, the laundry does not drop down and the stain on the laundry is not removed. The second prior art in which the rotor position is estimated from the motor current without using position sensors has a low limit of rotating speed required for estimating the position. Applied to a washing machine, such as the drum washing machine, which is driven at low rotating speed and accompanied with large change of load, the second prior art may cause a position estimation deviating from the actual value and may not be able to follow up the load fluctuations, resulting in going out of tune. In the second prior art, the motor is driven with vector control and load torque or other factors equivalent to it must be detected. As described above, the load changes suddenly depending on the action of the drop of the laundry, and thus it is hard to detect the load torque, causing the drive of the drum washing machine using the second prior art to be impossible.
Since the control apparatus of the sensorless DC motor in the third prior art is thus configured, when the load torque variation amplitude in rotation is steep and large, deviation of the current feed phase causes the lowered motor efficiency, vibration due to speed fluctuations, and out-of-tune.
Further no consideration has been given to detecting means of out-of-tune due to sudden load variations or protective means in case of oscillation.